I think our view of Sparta is influenced by two main things:
1. Spartan propaganda. It was in Sparta’s interest to portray themselves as crazy, devoted warriors who relished the thought of dying for their country. How would you feel going up against a bunch of guys who would rather die on the field of battle than go home in defeat and have their own mothers kill them.
2. We actually don’t have surviving first person Spartan accounts. Most of what we know about the Spartans comes from others- mainly the Athenians. Now the Athenians as bitter rivals to the Spartans also found it in their interest to portray the Spartans as crazy warrior psychopaths.
We know from history that the Spartans actually lost battles. Also in those battles, there were a lot of Spartan survivors who actually surrendered. In addition, we actually have no historical record of any mass killings of losing soldiers by their mothers.
So take what you read about Spartan warriors with a huge grain of salt.